As copper wires are used to electrically interconnect electrical components, optical fibers may be used to optically interconnect optical devices. The optical interconnection is quite simple when there is a one-to-one correspondence between optical devices. However, when the optical signal from a light emitting device must be directed to more than one receiving device, the optical interconnection is less than obvious.
One prior art suggestion is to use fiber optic bundles to split and direct the light from one emitting device to each one of a number of receiving devices. When there is a small number of devices, this approach may be readily implemented. However, as the number of emitting or receiving devices increases, the number of individual interconnections increases significantly, and this approach is commonly abandoned due to space considerations.
With the use of parallel processing techniques, it is often desirable to connect individually in a parallel manner the optical output of each device in one array of optical devices to the optical input of each device in another array of optical devices. As a result, the number of individual physical connections between the two arrays can be enormous. The total number of connections between two arrays equals the mathematical product of the number of optical devices in one array times the number of optical devices in the other array. Depending on the physical size of the devices as well as the interconnections, space considerations again can rapidly become a limiting factor. This is just one reason why optical parallel processing techniques have had such limited acceptance and use.
Another prior art approach for interconnecting optical devices is to use a computer-generated transmission hologram. Generally, a hologram consists of any material for storing the optical wavefront from an object that is encoded in an optical fringe pattern for subsequent recreation of the wavefront. One familiar example of a hologram for creating artistic visual effects is a photographic plate that has been exposed to coherent light from a three-dimensional object and a reference beam that interfere in the plate. The photographic plate is developed, and the reference beam is again passed through the developed photographic plate to recreate a three-dimensional image of the object.
One example of computer-generated transmission holograms for optically interconnecting the optical devices of an optical sequential logic system is described by A. A. Sawchuk et al. in Technical Report No. 1100 entitled "Nonlinear Real-Time Optical Signal Processing", University of Southern California Image Processing Institute, Los Angeles, Ca. 1983. The logic system includes a number of computer-generated Fourier transmission holograms positioned in a two-dimensional array for optically interconnecting a similar number of liquid crystal light valves also positioned in a two-dimensional array. The light valves are optically nonlinear optical devices in that the optical output signal from each device is a nonlinear gain function of the optical input signals applied to the device. As a result, the liquid crystal light valves are operated to regenerate optical signals and to perform an optical logic NOR function. However, one disadvantage of the liquid crystal light valve is that the optical input control signals are received on one surface of the device and that the optical output signals are emitted from another surface usually on the other side of the side. Thus, the transmission holograms and a complicated arrangement of precisely positioned lenses and mirrors must redirect the optical output signals from the rear surface of the light valve array 360 degrees onto the front surface of the light valve array. The long distance that optical output signals must travel from the rear surface of a light valve before being reflected as an input signal onto the front surface of at least one other light valve, severely limits the operating speed of any optical system using this transmission hologram interconnection arrangement. Another problem with interconnection arrangement is the mechanical precision required in aligning the transmission holograms and the light valves with the mirrors and the lenses. A slight vibration can misalign the entire system.
Another problem with Fourier transmission holograms is the significant power loss of an optical signal as it passes through the hologram. Each optical signal passing through a Fourier transmission hologram forms two images of which only one is used to interconnect the devices and has at most half the optical power of the incident signal. Furthermore, transmission holograms operate only with coherent light, which may result in optical interference at the input of an optical device due to constructive and destructive interference between the several input signals.